1. Field of the Invention
The present invention relates to an inkjet head and an inkjet recording apparatus, and more particularly to a technology for discharging ink through a nozzle driven with a piezoelectric element.
2. Description of the Related Art
In recent years, inkjet recording apparatuses (inkjet printers) serving as recording apparatuses that print-record images captured by digital still cameras or the like have become widely distributed. The inkjet recording apparatus is advantageous in that it is relatively inexpensive, simple to handle, and allows good quality images to be obtained. The inkjet recording apparatus has a plurality of recording elements in a recording head. The recording head is moved in a scanning direction while ink droplets are discharged from the recording elements to the recording medium such as recording paper, the recording medium is conveyed by one line when one line of image has been recorded on the recording medium, and an image is formed on the recording medium by repeating these steps.
There are inkjet printers that use a short serial head and record while causing the head to move in the width direction of the recording medium, or those that use a line head in which recording elements are arranged along a length corresponding to an entire width of the recording medium. In printers in which the line head is used, images can be recorded on the entire surface of the recording medium by scanning the recording medium in the direction perpendicular to the array direction of the recording elements. In printers in which the line head is used, a carriage or another conveyance system for moving the short head is unnecessary, and complex scanning control for the carriage movement and recording medium is not required. Moreover, the recording medium alone moves, so that recording speed can be increased in comparison with printers in which the serial head is used.
An example of a conventional print head is shown in FIG. 14. FIG. 14 is a cross-sectional view (corresponding to a cross-sectional view along line 4-4 in FIG. 3A) showing a three-dimensional configuration of an ink chamber portion inside the print head. Each nozzle 51 is provided with a pressure chamber 52. The pressure chamber 52 is substantially a square in a plane shape, and the nozzle 51 and a supply port 54 are provided to both corners on a diagonal line of the square. Each pressure chamber 52 is connected to a common flow channel 55 through each supply port 54.
An actuator 58 having a discrete electrode 57 is attached to a diaphragm 56 that constitutes the ceiling of the pressure chamber 52. The actuator 58 is deformed by application of drive voltage to the discrete electrode 57, so that the ink is discharged from the nozzle 51. When the ink is discharged, new ink is supplied to the pressure chamber 52 from the common flow channel 55 through the supply port 54. When a piezoelectric element is used as the actuator 58, the amount of ink droplets discharged can be controlled by a piezoelectric element drive signal (command signal), and the print speed can be increased by raising the drive signal frequency.
Japanese Patent Application Publication No. 5-212860 discloses an inkjet head, comprising: a pressure chamber formation portion having a nozzle substrate in which nozzle orifices are formed; a first substrate on which a reservoir and a pressure chamber are formed; an elastic plate on which an ink connection port for connecting to the reservoir is formed; a piezoelectric element disposed so as to be conjoined with the elastic plate wherein one end is fixed to an anchor block and the other end is provided as a free end; and a head frame for freely positioning the piezoelectric element via the anchor block and connecting the piezoelectric element with the pressure chamber formation portion on the upper face, wherein the inkjet head has a supply hole for feeding ink to the reservoir, and wherein the supply hole passes completely through the head frame in the direction that is parallel to the longitudinal direction of the piezoelectric element, whereby the piezoelectric element can apply longitudinal vibration to the pressure chamber. Thus, ink can be reasonably fed without wasting space in the width direction in a highly integrated inkjet head in which a plurality of pressure chambers are arranged at a high density, and even if bubbles have penetrated the ink supply holes, which are through holes, these bubbles can be easily removed.
However, in the inkjet recording apparatuses, the viscosity of the ink contained in the inkjet head changes depending on the environmental temperature, the service (resting) time, and other service environment factors. Hence, the inkjet head has to be able to discharge inks with various viscosities from low viscosity to high viscosity in order to maintain print quality. High viscosity ink is commonly difficult to discharge, and even if discharge is successful, its responsiveness is poor in comparison with low viscosity ink, and it becomes impossible to increase the operating frequency.
In the conventional example shown in FIG. 14, the nozzle (orifice) and the piezoelectric element face each other with the pressure chamber disposed therebetween. In such a structure, there is no option but to dispose the common flow channel in the space between the nozzle and the pressure chamber, and the distance between the nozzle and the piezoelectric element increases. Therefore, pressure applied by the piezoelectric element may not be transmitted to the nozzle position, and ink droplets with high viscosity might not be discharged. Nevertheless, this structure is required for separating the functions of the nozzle member and the piezoelectric element member.
Japanese Patent Application Publication No. 5-212860 does not disclose nor suggest the relationship between the pressure chamber formation portion and the discharge of ink droplets.